1. Field of the Invention
The present invention relates to a switching power supply, in particular, for reducing external parts for a control IC (i.e., a semiconductor integrated circuit) and manufacturing cost, and realizing downsizing.
Priority is claimed on Japanese Patent Application No. 2006-169830, filed Jun. 20, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 5 shows an example of a known voltage-pseudo-resonance switching power supply. In a method for reducing dependency of an overcurrent protection circuit in this type of switching power supply on DC (direct current) input voltage, when a switching device 110 is on, pulse-formed negative voltage V3 (i.e., forward voltage) is detected, which is generated at a drive winding 40 and is in proportion to DC input voltage V1 (see, for example, part (c) in FIG. 2). Based on the detected signal, DC input voltage correction for overcurrent protection is performed. Below, the relevant structure and operation will be explained with reference to the switching power supply shown in FIG. 5.
In FIG. 5, the primary control circuit includes a primary winding 20 of a transformer 10, which is connected to a DC input voltage V1; the drive winding 40; an activation resistor 50; a rectification smoothing circuit 60; a bottom control resistor 70; a bottom control capacitor 80; a start circuit (“START”) 90 for starting the operation; a constant voltage source (“REG”) 100; the switching device 110; a resonance capacitor 120 with respect to the primary winding 20; a drain-current detection resistor 130; a light receiving element 140a of a photocoupler; a phase control capacitor 150; a pseudo-resonance control circuit 160; a maximum on-width generation circuit 170; an output-stage circuit 180; and an input correction circuit 310.
The secondary circuit of the transformer 10 includes a secondary winding 30; a rectification smoothing circuit 210; an error amplification circuit 220; and a load 230.
The rectification smoothing circuit 210 rectifies and smoothes voltage, which is induced at the secondary winding 30 during the off-period of the switching device 110, by using a rectifying diode 211 and a smoothing capacitor 212.
The error amplification circuit 220 consists of a Zener diode 221, a resistor 222, and a light emitting element 140b of the photocoupler. An error signal of output voltage Vo on the secondary side with respect to reference voltage is fed back to the primary side via the light emitting element 140b and the light receiving element 140a of the photocoupler.
The switching device 110, the pseudo-resonance control circuit 160, the maximum on-width generation circuit 170, the output-stage circuit 180, and the like, are contained in a single control IC 300 (i.e., semiconductor integrated circuit) which is a constituent of the control circuit in the switching power supply.
After the switching device 110 is switched off and a specific flyback period is terminated, the pseudo-resonance control circuit 160 compares a voltage signal V4 with reference voltage V5 by using a bottom detection comparator 163, where the voltage signal V4 is obtained by shaping the (free oscillation) voltage V3 generated at the drive winding 40 by means of bottom control devices (i.e., the bottom control resistor 70 and the bottom control capacitor 80). When the voltage signal V4 falls below the reference voltage V5, it is determined that the free oscillation has reached its bottom, and an on-trigger signal having a Hi (i.e., high) level is output to the set terminal S of an SR flip-flop 164, thereby switching on the switching device 110.
While the switching device 110 is on, drain current I1 is converted into a voltage signal by the drain-current detection resistor 130, and an overcurrent detection signal V10, which has been shaped by a low-pass filter (LPF) 161, is compared with feedback voltage V11 supplied by the error amplification circuit 220 on the secondary side, by means of a feedback control comparator 162. When the overcurrent detection signal V10 has exceeded the feedback voltage V11, an off-trigger signal of Hi level is output to the reset terminal R of the SR flip-flop 164, thereby switching off the switching device 110.
An overcurrent detection comparator 166 is provided for determining an overcurrent state when the overcurrent detection signal V10 has exceeded reference voltage V13, and forcibly switching off the switching device 110 so as to protect it.
The maximum on-width generation circuit 170 is provided for preventing the on-width of the switching device 110 (i.e., a temporal width during which the switching device is on) from being excessively widened, when low voltage is input or the power is turned off.
While the switching device 110 is on, the input correction circuit 310 rectifies and smoothes the forward voltage V3, which is generated at the drive winding 40, via a diode 311, a resistor 312, and a capacitor 313. The rectified and smoothed signal is input (as the reference voltage) into the base of a PNP transistor 314 for clamping the feedback voltage V11. That is, control is performed in a manner such that the higher the DC input voltage V1, the lower the clamped feedback voltage (V11), thereby realizing overcurrent protection having small dependency on the DC input voltage.
In the conventional switching power supply shown in FIG. 5, the diode 311, the resistors 312 and 315, the capacitor 313, the PNP transistor 314, and the like, which are necessary for implementing the input correction circuit 310, are external parts of the control IC 300. In order to reduce the manufacturing cost and realize downsizing, removal of such external parts has been desired.